Evidence for orthologous QTL for plant height between maize and sorghum: verification with replicated progeny
--D. Ahnert, M.G. Pereira and M. Lee

RFLPs are being used to estimate the genetic locations and effects of QTL for plant height across generations and environments. In this note, we report the preliminary plant height QTL mapping results obtained by investigating F2:3 lines evaluated in one environment and compare the results to those obtained with F2 plants studied by Pereira and Lee (Theor. Appl. Genet. 1994). The F2:3 population was developed from a cross between Combine Kafir 60 (CK60) and PI229828. CK60 is an inbred line and a representatative of the subspecies bicolor. PI229828 is a wild-type representative of the Sorghum bicolor subspecies drumondii. They differ in several traits including panicle morphology, plant height, maturity, tiller production, and resistance to insects. One hundred and fifty-two F2:3 lines, derived by self-pollinating the F2 plants, were grown in a 12 x 13 rectangular lattice design of one-row plots with two replications near Ames, IA in 1994. A total of 8 traits were considered including plant height, peduncle size, panicle length, leaf length, leaf width, node number, tiller number, and stock diameter. These traits were also measured in the same location in 1993.

QTL were determined on the adjusted entry means at 111 loci by interval mapping using MAPMAKER-QTL and single-factor analysis of variance. Six independent QTL for plant height were identified in linkage groups A, B, D, E, F, and H. Individually, the QTL accounted for 11 to 32% of the phenotypic variation. The multiple QTL model accounted for 77% of the variation for plant height in this population. Additive effects ranged from 15 to 52 centimeters and the dominance effects from 5.7 to 123 centimeters. In all cases, alleles for increased plant height were derived from the tall parent (PI229828). Alleles from this parent were overdominant, except in linkage group H where the CK60 allele was partially dominant. As hypothesized by Pereira and Lee (Theor. Appl. Genet., 1994) the QTL of linkage group A, E and, H may correspond to the sorghum genetic loci Dw3, Dw4 and Dw2, respectively (Quinby and Karper, Agron. J. 46:211-216, 1954). The remaining QTL (group B) may correspond to the Dw1 locus or to additional loci influencing plant height not yet reported in sorghum.

The genetic locations of the QTL mapped with F2 plants coincide with the locations of the QTL mapped in the same linkage groups in the F2:3 population. Thus, the same genomic regions affecting plant height were identified across generations. The direction of the additive and dominance effects of these QTL were the same in both generations, but the magnitude of the effects differed. F2:3 progenies had slightly higher values for additive and dominance effects. This is probably due to the fact that F2:3 progenies were evaluated on a plot mean basis, which reduces environment variation and experimental error. Evaluation based on replicated progenies may also increase the efficiency of identifying QTL with small effects. This may have contributed to the identification of the two additional QTL for plant height mapped in the F2:3 generation (linkage groups D and F).

Comparative mapping in sorghum and maize has revealed that plant height QTL of sorghum linkage groups A, E, and H may be orthologous to plant height QTL identified for maize chromosomes 1, 6, and 9, respectively (Pereira and Lee, Theor. Appl. Genet., 1994). In both the F2 and F2:3 generations, the confidence intervals obtained from the sorghum QTL are within those obtained for maize (Beavis et al., Theor. Appl. Genet. 83:141-145, 1991, for QTL on chromosomes 1 and 9; Veldboom et al., Theor. Appl. Genet. 88:7-16, 1994, for QTL on chromosomes 1 and 6).

All measured traits will be considered in further analyses for consistency of QTL detection across generations and environments. 

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